An imaging system of this type, which is used more specifically for projection lithography methods in the semi-conductor industry is evident in WO 95/19637 (=U.S. Pat. No. 5,801,388) by the applicant.
Lithography represents an important step when structuring the semi-conductor substrates during the manufacture of semi-conductor components. The substrate, which for example can be a silicon wafer, is covered with a thin layer of light-sensitive material, a so-called photo-resist. A lithography imaging system is used to image a structure pattern onto the photo-resist; in addition to light or X-ray beams it is also possible to use particles in the form of a particle beam to expose the photo-resist. During the subsequent development step either the exposed or the non-exposed sites of the photo-resist are removed from the substrate. The substrate is then subjected to a procedural step, such as for example, etching, precipitation, oxidation, doping or the like, during which the pattern of the photo-resist on the substrate covers the particular sites of the surface which are not to be processed. After having removed the photo-resist, the substrate remains with the new structure. By repeating this sequence of steps it is possible finally to produce a succession of structure layers which form the desired semi-conductor structures, such as for example, the smallest switching circuits.
In the case of projection lithography systems which use a particle beam, stencil-like masks are used in which the patterns to be imaged are formed as orifices of an appropriate shape in a thin membrane of, for example, a few micrometers thick. The particles can only penetrate the mask orifices so that a beam pattern is produced which is projected, for example, in a reduced format on the substrate.
In addition to the ions primarily under consideration here, more specifically hydrogen ions or helium ions, it is also possible to use any other electrically charged particles including electrons a the particles for the lithography process. The advantages of using particles rather than light, are, for example, the considerably smaller wavelengths of the particles with the associated enhanced resolution and the greater depth of sharpness on the substrate.
Optical systems, regardless of whether they are light-optical or particle-optical systems, produce aberrations. It is known and general practise for particle-optical imaging systems to use electro-static lenses in the form of two or three rotationally symmetrical annular electrodes, which are formed as a tube, ring or diaphragm, where the beam passes through the middle of the said annular electrodes which lie at least partly at different electric potentials, or rather arrangements by combing such elements in rows. Lenses of this type always have a positive refracting power and are thus focussing lenses; furthermore without exception they have significant aberrations of the third order which can only be slightly influenced by the shape of the lens geometry.
By using diverging lenses (negative refracting power) it is possible to ensure that the aberrations produced by the arrangement of combined focussing lenses and diverging lenses are to a great extent compensated, the other coefficients of aberration are also maintained as small as possible. It is not possible by means of annular electrodes alone to achieve a lens of negative refracting power, such as for example from M. Szilagyi, "Electron and ion Optics", Plenum Press, 1988; on the contrary, it is necessary to use a plate or control grid electrode through which the beam passes.
EP 0 564 438 A1 (=U.S. Pat. No. 5,378,917) by the applicant discloses the use of a three-electrode lens in a particle-optical imaging system, wherein the three-electrode lens consists of two tube electrodes, between which there is located a control grid electrode so that the lens is divided by the control grid into two regions which are of different refracting power. In particular, one region can have a positive refracting power and the other can have a negative refracting power with a lower absolute value than the refracting power of the region with the positive refracting power, so that overall a lens which has a focussing effect is produced. The aberrations of the diverging region can be used to compensate for the aberrations of the focussing regions. By designing such a control grid lens in an appropriate manner it is possible to ensure that the electrical field strengths on both sides of the control grid electrode are equal with respect to their value and direction, as a result of which imaging interference, relating to the so-called aperture lens or fly's eyes effect, through the control grid orifices is avoided. Control grid lenses are, however, encumbered with the disadvantage that on the one hand the control grid is extremely thin but overall must have a large surface area and therefore is extremely sensitive to damage, all the more considering that the irradiation by particles, in particular ions, represents a considerable loading for the control grid. On the other hand, in order to avoid imaging interference it is necessary for the control grid to be moved sufficiently rapidly by a value equal at least to the width of one control grid cross piece in its plane and the requirement to maintain the precise position of the control grid in one plane places particularly high demands on the motion mechanics. It is therefore expensive to manufacture such a control grid lens and the control grid must be regularly inspected and maintained during use.
It follows from this that it is proposed in the above mentioned WO 95/19637 by the applicant that the control grid of the control grid lens is achieved by the mask foil itself which forms the middle electrode or the first electrode in the beam direction of the control grid lens. The mask foil formed in this manner comprises, likewise as is the case in EP 0 564 438 A1, the control grid lens a region of positive refracting power and a negative refracting power, the absolute value of the negative refracting power, however, being less than the positive refracting power, so that the total refracting power of the three-electrode lens is positive, i.e., focussing.
For the sake of simplicity reference is made to DE 197 34 059 A1 (=U.S. Pat. No. 08/914,070) by the applicant which discloses an arrangement for performing a shadow lithography method were a diverging lens is used as part of the illuminating system for the mask. In a preferred embodiment the mask itself forms the control grid of the diverging lens. In this shadow arrangement no projection system is provided downstream of the mask; on the contrary the substrate is disposed immediately behind the mask and the structures of the mask are imaged directly on the substrate. For this reason, owing to the lack of an optical system between the mask and the substrate the lithography arrangement of DE 197 34 059 A1 is to be regarded as not being of the same generic type as the subject matter of the present invention.
It is possible to achieve extraordinary image qualities both with respect to the resolution and also to the lack of distortion with the two embodiments described in WO 95/19637--mask as middle electrode or as a first electrode of a control grid lens. However, there is the disadvantage that the field strengths upstream and downstream of the mask are fairly different, so that a resulting force is produced which leads to a curvature of the thin mask foil. A further disadvantage resides in the fact that in both embodiments the mask is illuminated by a divergent beam of ions. In order to prevent the particles from inadmissibly scattering on the mask foil, which would cause image interference, the cross-section of the mask orifices through the foil must be tailored to suit the divergence of the beam. This means a complicated method of producing the mask which considerably increases the costs of such a system.
It is therefore an object of the invention to avoid these advantages which occur in an imaging system which uses an illuminating system and a projection system and at the same time to improve further the imaging characteristics, more specifically for the aberrations in the magnitude of 25.times.25 mm.sup.2 in the case of particle current strengths in the magnitude of 3 .mu.A to achieve resolutions below 100 nm.